Construction machines such as hydraulic excavators or the like are generally provided with a hydraulic pump, hydraulic actuators which are driven by hydraulic fluid delivered from the hydraulic pump, and flow control valves for controlling supply and discharge hydraulic fluid supplied to and from the hydraulic actuators. In case of a hydraulic excavator, for example, the hydraulic actuators include a boom cylinder for driving the boom of a front work implement, an arm cylinder for driving an arm, a bucket cylinder for driving a bucket, a swing hydraulic motor for swinging a swing structure, and a travel hydraulic motor for travelling a track structure. These actuators are combined with respective flow control valves. Each of the flow control valves has a meter-in restrictor and a meter-out restrictor. The meter-in restrictor controls the flow rate of hydraulic fluid supplied from the hydraulic pump to the corresponding hydraulic actuator, whereas the meter-out restrictor controls the flow rate of hydraulic fluid discharged from the hydraulic actuator to a tank.
On a construction machine equipped with such hydraulic actuators, the self-weight of an object supported by a hydraulic actuator (e.g., if the hydraulic actuator is an arm cylinder, then the arm or bucket (attachment) is a main object supported thereby) acts as a load, which may hereinafter be referred to as “negative load,” applied in the same direction as the actuation direction of the hydraulic actuator, tending to increase the actuation speed of the hydraulic actuator and hence causing a shortage of hydraulic fluid on the meter-in side thereby to bring about a breathing phenomenon (cavitation), which is likely to make the construction machine less controllable.
In respect of the above problem, the invention of Patent Document 1 discloses a circuit in which a pilot variable opening valve is inserted in a meter-out line branched off from the rod-side line of a hydraulic cylinder and connected to a tank such that the pilot variable opening valve is controlled to increase or reduce the opening thereof. In the circuit, when the actuation speed of an arm cylinder tends to increase due to the self-weight of an arm and a bucket which are heavy loads on the arm cylinder (tends to drop under self-weight), the opening of the pilot variable opening valve is restricted to prevent the holding pressure in the rod-side hydraulic chamber from decreasing, thereby preventing the arm cylinder from dropping under self-weight.